The same age as Okita Sōji and another member named Tōdō Heisuke, the three shared the distinction of being one of the youngest in Kondou Isami's group and being among its most gifted swordsmen. As a member of the Shinsengumi, Saitō was said to be an introvert and a mysterious person; a common description of his personality says he "was not a man predisposed to small talk." Saito was an unusually tall man at 5' 11". He was also noted to be very dignified, especially in his later years. He always made sure that his obi was tied properly and when he walked he was careful not to drag his feet. At rest he always sat in the formal position, called seiza, and he would remain very alert so that he could react instantly to any situations that might occur.
He was, however, known to be very intimidating when he wanted to be. Along with his duties as Captain of the Third Squad in the Shinsengumi, he was also responsible for weeding out any potential spies within the Shinsengumi ranks. Members had to constantly be mindful of what they said around him.
His original position within the Shinsengumi was assistant to the vice commander (副長助勤 fukuchō jokin?). His duties included being a kenjutsu instructor. Despite prior connections to Aizu, his descendants dispute that he served as a spy. His role as an internal spy for the Shinsengumi is also questionable; one common example being that he is said to have been instructed to join Itō Kashitarō's splinter group in 1867, to spy on them. However, this is disputed by Abe Jūrō, who did not believe he was a spy. It is probable that he also monitored other intelligence and enemy activity. His controversial reputation comes from accounts that he executed several corrupt members of the Shinsengumi; however, rumors vary as to his role in the deaths of Takeda Kanryūsai and Tani Sanjūrō.[4]
In the reorganization of the ranks in late 1864, he was first assigned as the fourth unit's captain. At Nishi Hongan-ji in April 1865 he was assigned as the third unit's captain. Saitō was considered to be on the same level of swordsmanship as the first troop captain Okita Sōji and the second troop captain Nagakura Shinpachi. In fact, it is rumoured that Okita feared his swordskill. Together with the rest of the Shinsengumi, he became a hatamoto in 1867. After the outbreak of the Boshin War (1868–1869), Saitō took part in Shinsengumi's fight during the Battle of Toba-Fushimi and the Battle of Kōshū-Katsunuma, before withdrawing with the Shinsengumi's survivors to the Aizu domain.
Due to Hijikata Toshizō being incapacitated as a result of the injuries sustained at the Battle of Utsunomiya Castle, Saitō became the commander of the Aizu Shinsengumi around May 26, 1868 under the name Yamaguchi Jirō (山口 次郎?) (which he had used since late 1867). After the Battle of Bonari Pass, when Hijikata decided to retreat from Aizu, Saitō parted with Hijikata and continued to fight with the Aizu army until the very end of the Battle of Aizu. This parting account was recorded in Kuwana retainer Taniguchi Shirōbei's diary, where it was recorded as an occurrence also involving Ōtori Keisuke, whom Hijikata requested to take command of the Shinsengumi; thus the said confrontation was not with Hijikata. However, questions regarding this parting remain, especially considering the conflicting dates.
Saitō, along with the few remaining men of the Shinsengumi who went with him, fought against the imperial army at Nyorai-dō (a small temple near Aizuwakamatsu Castle), where they were severely outnumbered.[5] It was at the Battle of Nyorai-dō that Saitō was thought to have been killed in action; however, he managed to get back to Aizu lines and joined the Aizu domain's military as a member of the Suzakutai. After Aizuwakamatsu Castle fell, Saitō joined a group of former Aizu retainers who traveled southwest to the Takada Domain in Echigo Province, where they were held as prisoners of war. In the records listing the Aizu men detained in Takada, Saitō is on record as Ichinose Denpachi.[6]